The Composite Output AOV node connects to the Output AOV Group node which is then connected to the Output AOVs pin on a Render Target node (figure 1). The Composite Output AOV node is used to assemble results of the AOVs composited using Composite Output AOV Layer nodes (figure 1).
Figure 1: The Composite Output AOV Node used to composite a DiffuseAmount of diffusion, or the reflection of light photons at different angles from an uneven or granular surface. Used for dull, non-reflecting materials or mesh emitters. Direct and Diffuse Indirect AOV together
Multiple layers can be added via the Add Layer button. Layers can be removed with the Remove Layer button. When a layer is added, a new input pin will appear to the left of the current layer pin(s).